Enhanced particle deposition system and method

ABSTRACT

A deposition system for depositing a chemical vapor onto a workpiece is disclosed, including a deposition chamber having a plurality of components for performing chemical vapor deposition on the workpiece. The workpiece is held by a lathe that rotates the workpiece relative to chemical burners that deposit silica soot on the workpiece. The deposition system has a gas panel for regulating the flow of gases and vapors into the deposition chamber, and a computer for controlling operation of the gas panel and the components in the deposition chamber. Multiple sets of chemical burners are disposed longitudinally along the length of the workpiece. Each set of burners is separated from other sets, such that each set of burners deposit silica particles onto generally different portions of a workpiece. The respective portions include an overlap segment in which one or more burners from one burner set will deposit silica particles on the same portion of the workpiece as one or more burners from another set.

TECHNOLOGICAL FIELD

The following disclosure relates to systems and methods for depositingchemicals onto workpieces, and the products therefrom. Moreparticularly, systems and methods for depositing silica soot on a startrod for fabricating optical fiber preforms, fused silica rods, and otheroptical components are disclosed.

BACKGROUND

Today's communications grade optical fiber of fused silica, SiO₂, ismanufactured according to three basic steps: 1) core preform or “startrod” fabrication, 2) core-with-cladding preform fabrication, and 3)fiber drawing. The core and cladding of a preform correspond in ratiosand geometry to those of the ultimate glass fiber that is drawn from thepreform.

The first step is to build up a start rod, forming it into a glass thatwill eventually become the fiber's core, and in some cases, also part ofthe fiber's cladding layer. The start rod is a glass rod made of silica,SiO₂, with the portion of the start rod that comprises the core beingdoped with a small amount of a dopant, typically Germania, GeO₂. Thepresence of the dopant in the core increases the refractive index of theglass material compared to the surrounding outer (cladding) layer. Inthe second step, a cladding layer is built up on the start rod. Theresult of this step is a preform having a core and a cladding, which isconventionally about 160 mm in diameter and about one meter long. Thethird step is fiber drawing, where the preform is heated and stretched,and typically yields about 400 km of optical fiber.

The primary raw ingredient to fabricating the glass preform is silicontetrachloride, SiCl₄, which generally comes in a liquid form. As notedabove, however, SiO₂, typically in the form of glass soot, is depositedon the start rod. The chemical reactions involved in the formation ofthe glass soot are complex, involving, SiCl₄, oxygen, O₂, and the fuelgas combustion products. In all of the techniques, the silica, SiO₂,comprises the cladding of the preform according, generally, to thereaction:SiCl₄+O₂═SiO₂+2Cl₂.

Generally, there are four distinct technologies for fabricating corepreforms. These technologies include Modified Chemical Vapor Deposition(MCVD), Outside Vapor Deposition (OVD), Vapor Axial Deposition (VAD),and Plasma Chemical Vapor Deposition (PCVD). The resulting product forall of these technologies is generally the same: a “start rod” that isgenerally on the order of one meter long and 20 mm in diameter. The coreis generally about 5 mm in diameter.

Similarly, there are generally four technologies for performing the stepof adding the cladding. These technologies include tube sleeving(conceptually paralleling MCVD), OVD soot overcladding (conceptuallyparalleling OVD), VAD soot overcladding (conceptually paralleling VAD),and plasma (conceptually paralleling PCVD). In this step, additionalcladding layers of pure or substantially pure fused silica are depositedon the start rod to make a final preform that can be prepared for fiberdrawing.

In MCVD, the step of manufacturing the start rod is performed inside ofa tube. Similarly, when the cladding step is performed, a larger tube issleeved onto and fused to the start rod. Presently, the company,Heraeus, manufactures tubes used for producing start rods and forsleeving onto and fusing with the start rods to make preforms.

In OVD, when fabricating start rods, glass is deposited onto a rotatingmandrel in a “soot” deposition process. The start rod is slowly built upby first depositing the germanium-doped core, and then the pure silicalayers. When the core deposition is completed, typically the mandrel isremoved and then the remaining body is sintered into a start rod ofglass.

In the process of OVD soot overcladding, where a cladding is depositedonto a fabricated start rod, the start rod is rotating and traversing ona lathe such that many thin layers of soot are deposited on the rod in astream from a chemical deposition burner over a period of time. Althoughthe SiO₂ is not deposited onto the start rod as a vapor, but rather asSiO₂ particles, the process is known in the art as a “chemical vapordeposition” process because the SiCl₄, which reacts in the streambetween the burner and the start rod to form SiO₂, is input to theburner as a vapor. The porous preform that results from the OVD sootovercladding process typically is then sintered in a helium atmosphereat about 1500° C., into a solid, bubble-free glass blank. U.S. Pat. No.4,599,098, issued to Sarkar, which is incorporated by reference asthough fully set forth herein, provides further background on systemsand techniques for OVD and OVD soot overcladding.

For the above-referenced technologies, typically any one of the corefabrication technologies may be combined with any one of the claddingfabrication technologies to generate a preform that may be used fordrawing fiber.

In the OVD soot overcladding processes, one of the key measures ofeconomic viability in comparison to the other available techniques isthe deposition rate of the SiO₂ on the workpiece. For example, somecompanies involved in optical fiber manufacturing opt for the mostcost-effective method of performing the step of overcladding the startrod in the fiber manufacturing process. With respect to this step in theprocess, the choice is either to purchase the cladding tubes or toperform a deposition process to add the cladding.

In considering the different approaches, the economics often are reducedto a question of whether a particular vapor deposition system that acompany is considering maximizes the deposition rate-to-cost ratio. Thedeposition rate may be characterized, for example, by the averagegrams/minute of silica soot that can be deposited on the start rod untilcompletion (i.e., an optical fiber preform ready for sintering). Above acertain average deposition rate, performing the soot overcladdingprocess is likely to be economically more attractive to the company thanpurchasing cladding tubes. Companies that manufacture systems forperforming soot overcladding focus on achieving the highest possibledeposition rates and being cost effective, but without compromising thequality of the preform that is produced for fiber drawing.

The factors that determine a deposition system's deposition rate are thechemical vapor delivery rate and the efficiency of chemical vapordeposition onto the workpiece. With respect to vapor delivery, keyissues generally revolve around continuously and efficiently maintaininga high (e.g., greater than 200 grams/minute) delivery rate over aprolonged period (e.g., greater than 2 hours). Several methods have beendescribed in the prior art for supplying a hydrolyzing burner with asubstantially constant flow of vaporized source material entrained in acarrier gas. For example, in U.S. Pat. No. 4,314,837 issued toBlankenship (“the Blankenship reference”), a system is described thatincludes several enclosed reservoirs each containing liquid for thereaction product constituent. The liquids are heated to a temperaturesufficient to maintain a predetermined vapor pressure within eachreservoir. Metering devices are coupled to each reservoir for deliveringvapors of the liquids at a controlled flow rate. The respective vaporsfrom each reservoir are then combined before they are delivered to theburner.

With respect to enhancing the deposition efficiency of SiO2 on theworkpiece to improve the effective deposition rate, studies have beenperformed to characterize the flow of chemical vapor from the burners tothe surface of the workpiece in the reaction chamber. One referencedirected to this issue is Li, Tingye, Fiber Fabrication, pp. 75-77,Optical Fiber Communications, (Academic Press, Inc. 1985). As discussedin the above reference, because of the small size of the formed glassparticles, momentum does not cause an impaction of the particles ontothe surface of the workpiece. The small sizes of the glass particleswould tend to force them to follow the gas stream around instead of atthe preform surface. Rather, the phenomenon of thermophoresis is thedominant mechanism for collection on the surface of the preform. As thehot gas stream and glass particles travel around the workpiece, athermal gradient is established near the surface of the preform.Preferably, the thermal gradient is steep, effectively pulling the glassparticles by a thermophoretic force towards the preform.

Various methods have been proposed to increase deposition efficiencybased on establishing and maintaining the thermophoretic force. Onemethod is to vary the distance between the burner and the workpiece. SeeH. C. Tsai, R. Greif and S. Joh, “A Study of Thermophoretic Transport Ina Reacting Flow With Application To External Chemical Vapor DepositionProcesses,” Int. J. Heat Mass Transfer, v. 38, pp. 1901-1910 (1995).Another set of methods, disclosed in U.S. Pat. Nos. 6,789,401, 7,451,623and 7,451,624 issued to Dabby et al., which are incorporated herein byreference as though fully set forth herein, involves selectivelytranslating the burners relative to the workpiece above variousthreshold velocities, e.g., greater than 1.4 meters per minute. Highervelocities mean that less heat is applied to any given spot on theworkpiece. The workpiece is therefore kept cooler, which tends toincrease the thermal gradient. Nevertheless, even applying thesemethods, demand for even higher deposition rates has gone unmet.

To further increase deposition rate, some have suggested providing anarray of numerous soot-depositing burners. These additional burners areproposed to be positioned for example, on a burner block along thelongitudinal axis of the lathe, where each burner deposits chemical sooton the workpiece. Specifically, U.S. Pat. No. 6,047,564 issued toSchaper et al., and incorporated herein by reference as though fully setforth herein, discloses a vapor deposition system in which a row oftwelve equally-spaced chemical burners is mounted on a burner block,each such burner depositing soot. The burner block moves forward andbackward along the longitudinal axis of the workpiece. The amplitude ofthe motion of the burner block generally corresponds to the distancebetween the burners such that each burner is deposits soot on adesignated segment of the entire workpiece.

Similarly, U.S. Pat. Nos. 5,116,400 and 5,211,732 issued to Abbott etal. and incorporated herein by reference as though fully set forthherein, disclose a deposition system comprising an array of chemicalburners. Like in the Schaper et al. patent, the Abbott et al. patentsdisclose a deposition process in which each burner in the chemicalburner array deposits soot on only a portion of the usable length of thepreform. The Abbott patents disclose an array of eleven burnerspreferably equally spaced from each other by about four inches. Theburner array is oscillated through a total distance 2J, with a distanceJ in each direction from the burner array's center position. The Abbottet al. patents disclose that preferably the oscillation amplitude isequal to or slightly greater than the burner spacing d in order toinsure uniformity of deposition. Accordingly, each burner traversesapproximately 20% of the length of the preform. In discussing varyingthe number of burners and their spacing to improve depositionefficiency, the Abbott et al. patents disclose that for itsconfiguration, deposition efficiency should improve as the number ofburners is increased.

Such multiple burner configurations as disclosed in the Schaper et al.and Abbott et al. patents are not commercially attractive in partbecause anticipated improvements in deposition rate have not beenrealized. The close proximity of the chemical burners to each othercompromises the thermophoretic effect such that the depositionefficiency significantly reduced. The close proximity of chemicalburners positioned over the length of a workpiece prevents regions ofthe workpiece from having sufficient time to cool before another burneris delivering soot on that same region. Furthermore, the amount of heatand number of chemical streams generated in the chamber caused by havinga large number of burners depositing soot compromises the desiredlaminar flow around the workpiece, which thereby reduces the neededthermal gradient for thermophoresis to occur. Without the optimaltemperature gradient between a burner and the workpiece, thermophoresisis weakened, which reduces the deposition efficiency, and thereby, theoverall deposition rate.

The costs associated with such multiple burner configurations are alsoprohibitive. These costs include not only the costs associated with theadditional chemical burners, but the costs of the vaporizers, preheatersand other equipment needed to support them, as well as the scrubbers andother equipment needed to handle the additional wasted depositionmaterial and heat that the burners produce. Furthermore, because theseburner configurations require vast amounts of chemical to achieveacceptable deposition rates, the cost of the chemical needed tomanufacture each preform is increased. These multiple-burnerconfigurations can therefore be fairly characterized as “brute force”approaches that are unduly wasteful of material and unnecessarilyexpensive.

As a result, a need exists for systems and methods that offer furtherimprovements to deposition efficiency, chemical delivery and, thereby,the overall deposition rate of chemical vapor. A need further exists forsystems and methods that offer cost effective manufacturing of opticalfiber preforms, including the manufacture of larger preforms in the samedeposition space, and accordingly, cost-effective optical fiber. A needfurther exists for multiple-burner configurations in chemical vapordeposition systems and processes that maintain the necessarythermophoresis to provide higher deposition rates and efficiencies.

SUMMARY OF THE DISCLOSURE

The following disclosure generally provides, in one aspect, systems andmethods for enhancing the effective deposition rate of chemicals onto aworkpiece, such as the deposition of SiO₂ from a SiCl₄ vapor onto astart rod for making a preform usable for drawing into optical fiber.

In a second separate aspect as described herein, systems and methods formanufacturing pure fused silica, optical fiber preforms, silica tubes,optical fiber, and silica rods, including fused silica rods, aredisclosed.

In a third separate aspect as described herein, a deposition system fordepositing silica particles onto a workpiece includes a first set ofburners for depositing silica particles onto a first portion of theworkpiece, a second set of burners for depositing silica particles ontoa second portion of the workpiece, and a lathe for holding the workpieceand for rotating the workpiece relative to the first and second sets ofburners. The first and second portions of the workpiece overlap eachother to form an overlap segment. Preferably, the longest distancebetween deposition burners within any set of two proximate burners isless than the shortest distance between burners in different sets.

In a fourth separate aspect as described herein, a method ofmanufacturing optical fiber includes the steps of obtaining a start rod,and depositing fused silica on the start rod to produce an optical fiberpreform. The depositing step includes steps of depositing silica on afirst portion of the start rod using a first pair of burners separatedfrom each other by a distance of about d, and depositing silica on asecond portion of the start rod using a second pair of burners separatedfrom each other by a distance of about d. The first and second portionsoverlap each other and the overlap preferably has a width of about d.Furthermore, the first pair and second pair of burners are preferablyseparated by a distance T, where T is preferably greater than threetimes d.

In a fifth separate aspect as described herein, a deposition system fordepositing silica particles onto a workpiece includes a first set ofburners for depositing silica particles onto a first portion of theworkpiece, a second set of burners for depositing silica particles ontoa second portion of the workpiece, a third set of burners for depositingsilica particles onto a third portion of the workpiece, and a lathe forholding the workpiece and for rotating the workpiece relative to thefirst, second and third sets of burners. The first and second portionsof the workpiece preferably overlap each other to form a first overlapsegment onto which one burner from the first set burners and a secondburner from the second set of burners substantially deposits silicaparticles. Similarly, the second and third portions of the workpiecepreferably overlap each other to form a second overlap segment ontowhich one burner from each of the second set and third set of burnerssubstantially deposits silica particles. In this configuration, thefurthest distance between deposition burners within any set of burnersis less than the shortest distance between burners in different sets.

In a sixth separate aspect as described herein, a method ofmanufacturing optical fiber includes the steps of obtaining a start rod,and depositing fused silica on the start rod to produce an optical fiberpreform. The depositing step preferably includes the steps of depositingsilica on first, second and third portions of the start rod using first,second and third pairs of burners, respectively. The burners in eachpair are preferably separated from each other by a distance of about d,where d is preferably greater than about 80 mm. The first and secondportions of the start rod overlap each other and the overlap has a widthof about d, and the second and third portions overlap each other andthat overlap also has a width of about d. In this configuration, theshortest distance between a burner of the first pair of burners and aburner of the second pair burners is greater than d, and the shortestdistance between a burner of the second pair of burners and a burner ofthe third pair burners is also greater than d.

Further embodiments as well as modifications, variations andenhancements are also described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a perspective view of a preferredembodiment of a chemical vapor deposition system;

FIG. 2 is a function-oriented diagram of a preferred embodiment of achemical vapor deposition system;

FIG. 3 is a diagram of a functional representation of a chemical burner;

FIG. 4 is a schematic diagram providing a perspective view of apreferred embodiment of a lathe, such as is generally depicted in FIG.2, for holding and moving a workpiece in a chemical vapor depositionsystem as is depicted in FIGS. 1 and 2;

FIG. 5 depicts a conventional configuration 500 of chemical burnersrelative to a workpiece;

FIG. 6A depicts a preferred embodiment of a multiple-burnerconfiguration 600 in a chemical vapor deposition process;

FIG. 6B depicts another preferred embodiment of a multiple-burnerconfiguration 600 in a chemical vapor deposition process; and

FIG. 7 is a process flow diagram illustrating a preferred embodiment ofa process of performing chemical vapor deposition such as may beperformed by the chemical vapor deposition system illustrated in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a preferred embodiment of a chemical vapor depositionsystem 100, usable in the manufacture of optical fiber preforms, opticalfiber, pure fused silica, fused silica tubes, core preforms, silicawafers, silica substrates and silica ingots. The chemical vapordeposition system 100 preferably includes a reaction or depositionchamber 102, and an enclosure 104 for the deposition chamber 102, acomputer 106 and other electronic components, an enclosure 108 for thecomputer 106 and the other electronic components, a power distributionsubsystem 110, an enclosure 112 for the power distribution subsystem110, a gas panel 114, a gas panel enclosure 116 and an intake andexhaust subsystem, including a main exhaust 118 and secondary exhausts122, 123.

The deposition chamber 102 is structured to house a process ofdeposition of particles (e.g., particles of silica soot) onto aworkpiece or start rod. The deposition chamber 102 and the depositionsystem 100 generally may be used to produce an optical fiber preformthat, in a subsequent drawing process, may be used to manufactureoptical fiber. The deposition system 100 may also be used to manufacturefused silica rods, including pure fused silica rods. For thisapplication, the deposition system 100 generally applies silica soot toan initial start rod of substantially pure fused silica. The product ofthis deposition process, a pure fused silica preform, is then sinteredto form the pure fused silica rod and may be used to manufacture (e.g.,by drawing, slicing or otherwise reforming the pure fused silica rod)silica wafers or substrates, multi-mode optical fiber, and other opticalcomponents for a variety of applications.

The other subsystems and components of the deposition chamber 102 aregenerally provided to support the deposition process. In one embodiment,deposition material generally comprising a vapor of silicontetrachloride (SiCl4) and oxygen (O2) is emitted from a chemical burnerin a process of fabricating optical fiber preforms in a depositionregion 146 of the deposition chamber 102. In the deposition region 146are the chemical burner, a chemical stream from the burner, and theworkpiece, where the stream is directed towards the workpiece from theburner. The burner also preferably issues and ignites streams ofhydrogen and oxygen. The resulting flame heats the chemical constituentsto temperatures generally exceeding about 1000° C. A chemical reactionwith the hydrogen, oxygen and SiCl4 occurs in the stream, in which theSiCl4 in the stream is oxidized producing particles of silicon dioxide(that are then deposited on the workpiece) and a byproduct of hydrogenchloride (HCl). Optionally, methane may be used to generate the heatnecessary in the deposition chamber 102, and octamethyl silica andsilane (octomethycyclotetrasiloxane) for a source of silicon to producesilicon dioxide.

The deposition chamber 102 preferably provides a laminar flow of air inthe deposition region around the workpiece. The provided laminar flowpreferably assists in maintaining a focused stream of heat and chemicalvapor from the burner towards the workpiece. A narrow and tight streamof flame enhances the thermophoretic effect that attracts the SiO₂particles to the workpiece because the SiO₂ particles get hotter whilethe surface of the workpiece remains relatively cooler.

Large quantities of oxygen (O₂) and fuel gas, typically in the form ofhydrogen (H₂) or natural gas, are passed through the deposition chamber102 to enable the deposition process of converting SiCl₄ into SiO₂ sootthat is deposited in layers onto a workpiece.

FIG. 2 is a diagram depicting a functional view of the chemical vapordeposition system 100 generally shown in FIG. 1. As depicted in FIG. 2,the chemical vapor deposition system 200 preferably includes a SiCl₄source 202, a nitrogen (N2) source 204, an oxygen (O₂) source 206, andan H₂ source 208 as raw materials for the vapor deposition system 200.Alternatively, the N₂, O₂, and H₂ sources 204, 206, 208 may be piped infrom an external location. The deposition system 200 preferably furtherincludes a computer 210, a gas panel 212, a preheater 214, and avaporizer 216 for controlling the flow of the materials used for thedeposition process.

The deposition system 200 preferably includes a deposition chamber orcabinet 218, enclosing preferably four or more chemical depositionburners 220, a lathe 222 for holding a workpiece 224 and for moving theworkpiece 224 rotationally and, optionally, translationally relative tothe four or more chemical burners 220. The deposition chamber 218preferably encloses one or more end-torches (not shown) positioned nearthe ends of the workpiece 224, and which preferably move (or remainstationary) with the workpiece 224. The end torches preferably directheat to the ends of the workpiece 224 to prevent it 224 from breakingand/or cracking Preferably, the workpiece 224 and the end torches moveso that the exhaust around the chemical burner 220 is maintainedrelatively constant. Alternatively, the chemical burners 220 are movingand the workpiece 224 and end torches are stationary (except for therotation of the workpiece 224). The deposition system 200 preferablyfurther includes an air intake and exhaust subsystem 226 includingscrubbers (not shown).

The computer 210 preferably includes electronic connections to thevaporizer 216, the gas panel 212, and the deposition cabinet 218 forautomatically controlling functions of each component. The computer 106,210 preferably further includes a connection to a user-input device suchas a keyboard, touch screen, knobs, buttons, switches, mouse and/ormicrophone for voice activated command input for providing operationalcontrol of the deposition system 200 to a user. Moreover, the computer210 preferably includes a user output device, such as a display monitoror speaker for presenting a status of the system.

The raw deposition materials' sources 202, 204, 206, 208 are preferablyreservoirs, which may be commercially available pressurized tanks forcontaining each constituent material. The SiCl₄ preferably is containedin a reservoir in liquid form, preferably at room temperature. The SiCl4source 202 preferably is connected by a pipe or line to the preheater214, such that SiCl4 may be conveyed as a liquid into the preheater 214.Preferably, positioned above the SiCl₄ source 202 is an exhaust port 203to convey SiCl₄ to a pollution control system (not shown) in event of aleak of SiCl₄ from its source 202. The preheater 214 is connected to thevaporizer 216 for transferring the heated SiCl4 liquid out of thepreheater 214 and into the vaporizer 216.

The vaporizer 216 comprises a container for containing a substantialvolume of SiCl₄, a heating element to heat the SiCl₄ in the container,and numerous valves (not shown) to regulate the flow of materials intoand out of the vaporizer 216. The vaporizer 216 is preferablyelectronically connected to the computer 210. Through this electronicconnection, the volume of SiCl₄ in the vaporizer 216 is preferablyregulated and maintained between a predetermined minimum and maximumlevel. The computer 210 preferably controls the flow of SiCl₄ liquidfrom the SiCl₄ source 202 to the vaporizer 216 from a solenoid valve217. The vaporizer 216 is also preferably pneumatically connected by aline to the N₂ source 204. Through control from the computer 210, theSiCl₄ source 202, the preheater 214 and the vaporizer 216 preferablyprovide a constant, automatic and prolonged flow of vaporized SiCl₄ fromthe vaporizer 216 to the burners 220 in the deposition cabinet 218.

The gas sources 204, 206, 208 are preferably pneumatically connected tothe gas panel 212. The gas panel 212 includes valves and mass flowcontrollers to regulate the flow of gasses from the gas sources 204,206, 208. Control of the valves in the gas panel 212 is provided by thecomputer 210, which is electronically connected to the gas panel 212.Lines for O₂ and H₂ are provided to preferably pneumatically connect thegas panel 212 and the burner 220 in the deposition cabinet 218. Further,a separate line is preferably provided to convey O₂ to the line carryingthe vaporized SiCl₄ to the burner 220. Thus, at a “T” fitting 219, thevaporized SiCl₄ and O₂ are mixed, and continue as a mixture in theirtransport to the burners 220.

Accordingly, four separate lines are preferably input to the burners220: a line conveying a mixture of vaporized SiCl₄ and O₂, a lineconveying H₂ or another convenient fuel gas, a line conveying O2 for thecombustion of hydrogen, and a line conveying O₂ to shield the SiCl₄ andO₂ mixture. This configuration preferably assures at least close to thesame volume of SiO₂ particles from each of the four chemical burners 220at any given point in time.

FIG. 3 depicts a preferred embodiment of a burner 300 for use in thedeposition system. The burner 300 preferably receives the four streams(one for each input line), and emits preferably four streams from aburner face 302, each stream being emitted from one of at least fourconcentric rings 304, 306, 308, 310 of emission holes.

As the constituents are emitted from the burner 300, the fuel gas andthe oxygen are ignited. The SiCl₄ particles react in the flame at acontrolled distance away from the face of the burner 300. The SiCl₄particles passing through the flame are oxidized to form silica sootthat continue in a directed stream toward a workpiece 224 that mayinitially be in the form or a start rod. As silica soot approaches theworkpiece 224, the silica soot has a temperature on the order of about1100° C. The chlorine is preferably separated from the other materialsand combines with hydrogen to ultimately form hydrochloric fumes (HCl).These reactions generally apply to the deposition process for a claddingon an optical fiber preform. Other constituents may be used for chemicalvapor deposition for other applications applying the differentembodiments and aspects of the chemical vapor deposition systemdescribed herein.

Referring again to FIG. 2, the silica soot is deposited in layers on acontinuously moving workpiece 224. The workpiece 224 is mounted on thelathe 222, which preferably rotates and translates the workpiece 224relative to the burners 220. As shown in FIG. 4, the lathe 400preferably includes end holders 402 into which the ends of the workpiece224 (e.g., the start rod) are inserted. The lathe 400 further includesat least one and optionally two motors 404 and 406 for moving theworkpiece 224 relative to the burners 220 both rotationally, andoptionally, translationally. The motors 404, 406 are preferablycontrolled by a computer 210, such as that depicted in FIG. 2, forcontrolling the speed of rotation and translation of the workpiece 224throughout the course of the deposition process on the workpiece 224.

In the deposition process, the translation speed of the workpiece 224relative to the burner may alternate (e.g., velocity to the right versusto the left) between slow (e.g., 1 meter per minute) and fast (e.g.,more than five meters per minute). The fast pass may be performed sothat the deposition at the end of a slow pass (e.g., at the left endportion of the workpiece), effectively resumes with a slow pass thatcommences at the right end-portion of the workpiece. Such a motionprofile may be advantageous because at the conclusion of a slow pass tothe left, the thermophoretic force will be strongest where the workpieceis the coolest, which, in this example, would be at the right end, whichhas been afforded the most time to cool. A very fast pass (e.g., greaterthan 20 meters per minute) may be employed primarily for the purpose ofquickly positioning the burners for another slow pass that starts at thecoolest point on the workpiece. This approach also helps to minimize the“footballing” effect that is discussed in the Dabby et al. patentsreferenced herein.

FIG. 5 depicts a conventional configuration 500 of chemical burnersrelative to a workpiece onto which silica soot is deposited. Two burners502 are separated by a distance d and traverse a longitudinal pathrelative to a workpiece 504. The configuration also preferably includestorches (not shown) that heat and solidify the ends of workpiece toprevent it from bending or cracking. The torches are distinguished fromthe burners, in that the torches do not typically deposit soot on theworkpiece. In the deposition process, the pair of burners 502 traversesa distance T along the longitudinal length of the workpiece 504, whilethe workpiece is rotating on a lathe. The pair of burners 502 reachesthe endpoint 503 of a traversal in one direction. Then, the pair ofburners 502 returns along the essentially same longitudinal path in theopposite direction. Over the course of a deposition process, theendpoint 503 may vary slightly from pass to pass, to avoid producing arippling effect on the surface of the preform. At the endpoints 503 ofthe traversal, a portion of the workpiece having a width ofapproximately d, where d is the distance between the chemical burners502, receives deposition from one of the two burners 502 in the pair,but not both burners 502. As a result of having only one burner depositsoot onto the ends of the workpiece, any tapering of the workpiece atits endpoints may be amplified, as depicted by the tapered regions 506.

A configuration of two burners in relatively close proximity is suitablefrom the standpoint of thermophoresis because two burners so configuredtypically will not deposit on the same region of the workpiece. Duringthe deposition process, because the workpiece is continuously rotating,the second burner in a pair will typically deposit on the radial side ofthe workpiece that opposes the side on which the first burner in thepair has deposited. Once a third close-proximity burner is added to theburner configuration, thermophoresis is impacted because the thirdburner will typically generally deposit on the same region on which thefirst burner deposited soot. Because of the third burner's closeproximity to the other two burners, that region of the preform has notyet had sufficient time to cool for thermophoresis to adequately takeplace.

FIG. 6A depicts a multiple-burner configuration 600 in a chemical vapordeposition process that overcomes the prior art's shortcomings of aweakened thermophoretic force as a result of using three or more burnersin close proximity to each other while providing an enhanced depositionrate through the use of additional burners. In the new multiple-burnerconfiguration, thermophoresis is not materially compromised, leading toenhanced deposition efficiency and greatly enhanced deposition rate, aswell as a practical use of deposition material. FIG. 6A depicts a firstset of chemical burners 602 and 604 and a second set of chemical burners606 and 608. The burners in each set are preferably spaced from eachother by a distance d1 (e.g., 80 mm-150 mm), and d1 is preferably thesame for both sets of chemical burners. The mean distance between setsof burners is T, where T preferably is a distance (e.g., 40 cm)sufficient for thermophoresis to take place when one pair of burnersdeposits on a region of the preform following a deposition on thatregion by the other pair of burners. To establish a consistentthermophoretic force, the distance between pairs of burners must beabove a certain threshold so that enough time elapses betweendepositions by the different pairs of burners. Preferably, the distance,T, between pairs of burners is generally greater than (e.g., threetimes) d₁. In this configuration, the distance T is also approximatelyequal to the distance that the burners 602, 604, 606, 608 traverselongitudinally in each direction. The value of T represents only aportion of the length, L, of the workpiece 610, such that each of thetwo sets of burners is responsible for depositing on approximately halfof the workpiece 610. In the middle of the workpiece 610, both sets ofburners responsible for an overlap segment 612, with each set of burnerscontributing to the deposition generally in accordance with thetriangles shown in FIG. 6A. In this multiple-burner configuration, L isgenerally equal to about 2 T+d. In FIG. 6A, each portion of theworkpiece 610 for which a set of burners is responsible is shown, purelyfor clarity of explanation, separately. Of course, in practice the twoportions form one continuous workpiece 610.

In the deposition process, the chemical burners generally remain fixeddistances from each other while they traverse the workpiece 610. Forexample, while chemical burners 602 and 604 traverse to the right alongthe left side of the workpiece 610, chemical burners 606 and 608traverse to the right along the right half of the workpiece 610.Similarly, while chemical burners 602 and 604 traverse to the left alongthe left side of the workpiece 610, chemical burners 606 and 608traverse to the left along the right half of the workpiece 610. In thismanner, each set of burners do not interfere with the other set as theydeposit soot on the portion of the workpiece 610 for which each isresponsible.

In the burner configuration depicted in FIG. 6A, each of the fourchemical burners 602, 604, 606, 608 preferably streams about the sameamount of silica soot at any given point in time. Indeed, both sets ofburners preferably receive chemical from the same supply, so as to helpinsure that both sets of burners stream the same volume of silica sootat any given time. At any given point on the workpiece 610, generallytwo burners are capable of depositing silica soot on the workpiece 610,except at the ends of the workpiece 610, where at the left end-portion,only the leftmost burner 602 will deposit silica soot, and at the rightend-portion, where only the rightmost burner 608 will deposit soot.However, between the end-portions, two burners deposit silica soot onthe workpiece 610. At the interface of the deposition between the twosets of burners, burner 604 of the left burner set and burner 606 of theright burner set deposit silica soot, so that even at the interface, twoburners are depositing silica soot on the workpiece. Such a depositionenables the creation of a preform with generally a seamless transitionbetween the portions of the preform formed by the depositions from eachrespective set of chemical burners. The result of this process is theformation of a preform with a fully usable portion, A, as depicted inFIG. 6A between the end-portions of the workpiece 610.

FIG. 6B depicts another multiple burner configuration 640 that overcomesthe prior art's shortcomings of a weakened thermophoretic force whenusing multiple burners. In this alternative multiple-burnerconfiguration, thermophoresis is not compromised, which accordinglyprovides enhanced deposition efficiency. FIG. 6B depicts a set ofchemical burners 642 and 644, a second set of chemical burners 646 and648, and a third set of chemical burners 650 and 652. The burners ineach set are preferably spaced from each other by a distance d1 (e.g.,80 mm-150 mm), and d1 is preferably the same for each set of chemicalburners. The mean distance, T₁, between the first set of burners and thesecond set of burners is preferably the same as the mean distance, T₂,between the second set of burners and the third set of burners, suchthat T₁=T₂. Also, shortest distance between burners in different sets islonger than the longest distance between deposition burners in the sameset. The parameters T₁ and T₂ are distances (e.g., 30 cm) generallysignificantly greater than (e.g., 3 times) d₁, and T₁ and T₂ representonly a portion of the length, L, of the workpiece 654 such thatpreferably L is equal to about 3 T₁+d₁ (or 3 T₂+d₁). As with theprevious configuration, the longitudinal distance traveled by theburners in each direction relative to the workpiece is equal to aboutthe mean distance between nearest sets of burners.

In the deposition process, the burners generally remain these fixeddistances from each other while traversing the workpiece 654. Forexample, while chemical burners 642 and 644 traverse to the right alongthe leftmost portion of the workpiece 654, chemical burners 646 and 648traverse to the right along a middle portion of the workpiece 654 andchemical burners 650 and 652 traverse to the right along the rightmostportion of the workpiece 654. Similarly, while chemical burners 642 and644 traverse to the left along the left side of the workpiece 654,chemical burners 646 and 648 traverse to the left along the middleportion of the workpiece 654, and chemical burners 650 and 652 traverseto the left along the rightmost portion of the workpiece 654. In thismanner, the three sets of burners do not interfere with each other whiledepositing on the portion of the workpiece 654 for which each set isresponsible.

In this configuration, each of the six chemical burners preferablystreams the same amount of silica soot at any given time. Again, as inthe previous embodiment, the three sets of burners preferably receivechemical from the same supply. At any particular point on the workpiece654, generally two burners will deposit silica soot on the workpiece654, except at the ends of the workpiece 654, where at the leftend-portion 656, only the leftmost burner 642 will deposit silica soot,and at the right end-portion 658, the rightmost burner 652 will solelydeposit on that portion of the workpiece 654. However, between theend-portions, two burners deposit silica soot on the workpiece. At theinterfaces 660, 662 of the deposition between each set of burners, twoburners generally will deposit soot. At interface 660, burner 644 of theleft burner set and burner 646 of the middle burner set deposit silicasoot, so that at the interface, two burners are depositing silica sooton the workpiece 654. Similarly, burner 648 of the middle burner set andburner 650 of the right burner set deposit silica soot, so that at thatinterface 662, two burners are depositing silica soot on the workpiece654. Such a deposition enables the creation of a preform with generallya seamless transition between the portions of the preform formed by thedepositions from each respective set of chemical burners. The result ofthis process is the formation of a preform with a fully usable portion Aas depicted in FIG. 6B between the end-portions of the workpiece.

In an alternative configuration, four burners such as burners 602, 604,606, and 606 of FIG. 6A may be used to produce a preform of length L,where L is equal to about 3 T+d₁, as depicted in FIG. 6B. In thisconfiguration, the four burners travel relative to the workpiece adistance of preferably about 2 T.

In the design of a deposition system, there are different issues andconstraints associated with the choice of whether the workpiece isphysically translated to pass in front of stationary (or nearlystationary) burners or whether the burners are translated to pass alonga stationary (or nearly stationary) workpiece. In the case of physicallymoving burners, serious (generally surmountable but costly) issues existrelating to maintaining a consistent exhaust of heat and depositionmaterial from the deposition chamber due to the fact that the sources ofthe heat and deposition material are moving within the chamber. On theother hand, physically translating the workpiece relative to stationaryburners causes the anticipated size of the workpiece and the distance ofits translation within the chamber to define the minimum length of thechamber itself. A longer and bigger chamber not only has a largerfootprint, it is also more expensive to manufacture. Although bothalternatives have tradeoffs, manufacturers typically design thedeposition chamber to provide for some amount of translation of theworkpiece so as to minimize exhaust and other design issues associatedwith having moving deposition burners. Using the burner configurationsdisclosed herein, the minimum length of the deposition chamber may bereduced, because the necessary translation distance of the workpiece issignificantly reduced in comparison with conventional configurations,such as the configuration depicted in FIG. 5. For example, using theconfiguration of FIG. 5, a two-meter workpiece would have to betranslated two meters in each direction to allow for the pair of burners502 to deposit on the entire workpiece. As a result, the translationdistance and length of the workpiece would effectively require a chamberhaving a length of no less than four meters.

In contrast, using the configuration of FIG. 6A having two pairs ofdeposition burners, a two-meter workpiece may only have to be translatedone meter in each direction. The translation distance and length of theworkpiece would effectively reduce the minimum length of the depositionchamber three meters. For the configuration of FIG. 6B, which has threepairs of deposition burners, the two-meter workpiece would require atranslation of approximately one half-meter in each direction. Thetranslation distance and length of the workpiece would effectivelyreduce the minimum length of the deposition chamber 2.5 meters.Conversely, given a predetermined length of a deposition chamber, themaximum lengths of the preforms that can be manufactured within it areincreased by implementing the new burner configurations disclosedherein.

FIG. 7 depicts a preferred embodiment of a process 700 of performingchemical vapor deposition such as may be performed by the chemical vapordeposition system 100 illustrated in FIG. 1. Optionally, in a first step702, a start rod is obtained. In the deposition process, a length for astart rod is set. In different runs of the vapor deposition system,start rods of various lengths, preferably between about 0.8 meters andabout 4 meters, may be used. Preferably, a length of a start rod isinput at an operator terminal and transmitted to a computer. Thecomputer then communicates with components of the deposition system thathave functions dependent on the start rod length. Specifically, thelathe may be programmed according the length of the start rod that isused for a particular run of the deposition process 700. When the lathereceives the length value from the computer, the torch at one end of thelathe is preferably automatically repositioned to apply heat to one endof the rod. The other torch is preferably stationary. Furthermore, themotor controlling the translation of the rod executes a traverse motionprofile that reflects the entered length of the start rod.

In a next step 704, silica soot is deposited in passes along aworkpiece, with each of the two, three or more sets of burners(depending on the maximum possible length of the workpiece) depositingsoot on respective portions of the workpiece, including the overlapsegments. The first pass of depositing silica soot preferably isperformed with a high flow of fuel gas and oxygen from the chemicalburner relative to the flow of SiCl₄. Furthermore, on this first passthe traverse speed is relatively low (e.g., one meter per minute). Theresulting high heat of the soot stream and of the workpiece on thisfirst pass hardens the initial interfacial layers between the start rodand the cladding layers that are subsequently deposited, preferablypreventing interface defects and slippage of the soot over the startrod.

During the first pass and throughout a run of the deposition process700, certain parameters are preferably fixed throughout the run.Specifically, the end torches at each end of the start rod provide aflame that preferably provides a source of heat. The end torches provideheat at the ends of the workpiece to prevent the soot from cracking andto eliminate the soot slippage over the start rod during sintering, bykeeping the ends denser and tightly adhered to the handle glass. The rodshould be hot enough to affix the ends of the soot to a particular pointon the start rod. However, if the end burners provide too much heat,then generally the start rod bends. Furthermore, to enhance theeffective deposition rate, the distance between the burners/torches andthe workpiece is preferably permitted to close as the workpieceincreases in diameter throughout the run. This closing of the distancebetween the burners/torches and the workpiece via the workpiece'snatural increase in diameter typically effectively meets the need forincreasing the amount of heat applied to the workpiece due to itscontinuously increasing size and mass. Thus, as the workpiece increasesin diameter, the chemical burners preferably remain stationary.

In the deposition process, and in accordance with the embodimentsdiscussed herein, one set of burners deposits soot on one portion of theworkpiece while at the same time another set of burners deposits onanother portion of the workpiece. Throughout the deposition process, thesets of burners remain generally equidistant from each other, and indeedmay be affixed to the same burner block which may perform thetranslation of the burners relative to the workpiece. Via such aconfiguration where the sets of burners are relatively distant from eachother, when one burner has deposited on the overlap portion, sufficienttime passes before a burner of the other set deposits on that sameportion. That time allows the overlap segment to cool sufficiently tofacilitate a strong thermophoretic force each time deposition isperformed on that segment.

Once the deposition process is completed to form an optical fiberpreform, in a next step 706, the preform is sintered in a furnace andthen drawn in a next step 708 into optical fiber. In a next step 710, aprotective coating preferably comprised of acrylate is applied to thedrawn optical fiber, which is then preferably UV-cured to harden thecoating. In a final step, the protected optical fiber is generallyplaced into tubes that may hold 100 or more optical fibers to form anoptical fiber cable that is ready for use in telecommunicationsapplications.

While preferred embodiments of the invention have been described herein,and are further explained in the accompanying materials, many variationsare possible which remain within the concept and scope of the invention.Such variations would become clear to one of ordinary skill in the artafter inspection of the specification and the drawings. The inventiontherefore is not to be restricted except within the spirit and scope ofany appended claims.

What is claimed is:
 1. A deposition system for depositing silicaparticles onto a workpiece, the deposition system comprising: a firstset of chemical deposition burners for depositing silica particles ontoa first portion of the workpiece, the first set having a first chemicaldeposition burner and second chemical deposition burner; a second set ofchemical deposition burners for depositing silica particles onto asecond portion of the workpiece, the second set having a third chemicaldeposition burner and a fourth chemical deposition burner; a lathe forholding the workpiece and for rotating the workpiece relative to thefirst set and second set; and a computer coupled to the lathe to controlthe position of the workpiece relative to the sets; wherein the longestlongitudinal distance between the chemical deposition burners withineach of the sets is less than the shortest longitudinal distance betweenthe chemical deposition burners in different sets; and wherein thecomputer is configured to, based on the length of the workpiece, causerelative movement between the workpiece and the sets such that, along alongitudinal path in one direction: only one of the chemical depositionburners from the first set traverses a first end segment; one of thechemical deposition burners from the first set traverses an overlapsegment in which the first portion and the second portion overlap; oneof the chemical deposition burners from the second set traverses theoverlap segment; and both of the chemical deposition burners from thefirst set traverse a medial segment of the first portion that extendsfrom the first end segment to the overlap segment.
 2. The depositionsystem of claim 1, the first and second chemical deposition burnersbeing spaced approximately a longitudinal distance d from each other,the third and fourth chemical deposition burners being spacedapproximately a longitudinal distance d from each other, a meanlongitudinal distance, T, between first set and the second set isgreater than 3×d, and the overlap segment has width of about d.
 3. Thedeposition system of claim 1, wherein a length of the workpiece L isgreater than about 80 cm, and d is between about 80 mm and about 150 mm.4. The deposition system of claim 3 wherein L is greater than 2×T. 5.The deposition system of claim 1 wherein the system is configured suchthat, during a portion of the deposition, the first, second, third, andfourth chemical deposition burners will receive chemical from a commonsource, and stream soot at approximately the same rate.
 6. Thedeposition system of claim 1, wherein the total number of chemicaldeposition burners is four.
 7. The deposition of claim 2, wherein thedeposition system is configured to cause relative longitudinal movementbetween the workpiece and the first and second sets of burners thatreciprocates along a longitudinal distance of travel that is equal to T.